First of all so sorry for being a little lazy with my English in the posts, but the neglecting will hopefully end really soon. There are just a couple of deadlines and the necessary Christmas Stress right now that have been keeping me a little too busy. But when I was asked today how you'd create a header that lies behind the content, I wanted to write it for you.English Deutsch

The CSS is very simple. What we do is fix the navigation to the top with a z-index: 2. This will put it above everything.
For the header we'll define a background image, which is attached fixed as well. I gave mine a height in px, but if you want your header in a specific aspect ratio, check out my post about the padding hack.
The text will be placed on the header. And then we'll give the whole header a z-index of -1.
In order for the header to be covered on scroll, all we need to do after is give the section a background color.